Suppose you know X. How do you know? Maybe you know because of Y. How do you know Y? Maybe the answer is Z. How do you know Z?

This is the regress problem of knowledge, also called the Agrippan trilemma and the Münchhausen trilemma. It is based on the supposition that if we claim to know something, we must have a reason for it and that reason must itself be something that we know. This leaves open four possible solutions. One is skepticism, the belief that we have no knowledge. The most common is foundationalism, which posits certain basic facts that require no external reasons to be justified. Another option is coherentism, which solves the problem via a kind of circular reasoning or justification loop. And finally, there is infinitism, the view that there is no end to the regress. For any chain of justification, the final member of the chain will always be unjustified, and it is always possible to go looking for further reasons of reasons of reasons. As infinitist Peter Klein puts it, knowledge is never "settled". Even so, says Klein, it is still possible to have knowledge. In this interview, Klein first argues why he thinks coherentism, foundationalism, and a certain kind ofskepticism all fail. He then explains his own account of justification, as "something that we do", and how it makes the infinitist picture look more plausible than it first seems. Along the way, he recounts the framing of the problem by Sextus Empiricus in his book Outline of Pyrrhonism. Klein argues that the ancient Greek Pyrrhonians, though they called themselves skeptics, were really infinitists.